Injection systems, such as common-rail injection systems for example, are used to inject fuel into an internal combustion engine, for example into a diesel engine or into a spark-ignition engine of a vehicle. To this end, an injection system has, as is known, one or more injection valves which can be actuated by means of a control unit of the injection system in order to inject the fuel. An injection valve, often also called an injector, has a blocking element, which can be moved by means of a drive of the injection valve, in order to open and close the injection valve. If the blocking element is in a blocking position, the injector is closed and no fuel will be injected. If the blocking element is not in the blocking position, the injector is partially or completely open and fuel will be injected. The injector usually has one or more injection openings through which the fuel will be injected and which, to this end, can be partially or completely unblocked or closed by means of the blocking element.
In order to minimize exhaust gas and noise emissions and in order to achieve a level of energy efficiency which is as high as possible, it is important for it to be possible for injection quantities of the fuel to be metered in as accurate a manner as possible and for the time points of the injection operations to be controlled in an accurate a manner as possible. In particular, it is known that an injection operation can comprise one or more individual injection operations, such as a main injection operation and further pre- and/or post-injection operations. Therefore, a non-trivial injection rate profile of an injection operation over time can be achieved by using a plurality of individual injection operations. An injection rate is to be understood to mean the quantity of fuel injected per unit time.
To this end, provision is often made for the injection valve to not open completely but rather only partially, that is to say for a reduced degree of opening of the injection valve to be set. In order to partially open the injection valve, the blocking element is not moved away from the blocking position as far as a mechanical stop, but rather, for example, only until some, but not all, of a plurality of injection openings are unblocked. This is possible, for example, when the injector is a so-called “variable nozzle”, the injection holes of said variable nozzle being arranged in different planes (“levels”). A reduced degree of opening can also be achieved by a throttling effect of the blocking element, said throttling effect being all the greater the closer the blocking element is to its blocking position. If the blocking element of the injection valve has a nozzle needle for blocking one or more injection openings, a reduced degree of opening of the injection valve can therefore be achieved by the nozzle needle not being lifted completely and out of the blocking position as far as a stop of the nozzle needle, that is to say as far as a maximum needle stroke of the nozzle needle, but rather that said nozzle needle is lifted only as far as a reduced needle stroke.
In addition to the described process of forming the time profile of injection rate, it is also advantageous to know the position of the blocking element for vehicle diagnosis operations (“on-board diagnostics”) by means of an engine controller or an “on-board unit” (“OBU”). Therefore, injectors which have, for example, failed, are always closed or are always open can be diagnosed, for example an electrical defect in one of the injectors of the injection system can be diagnosed. Information relating to the position of the blocking element can also be used to draw conclusions about the injection rate and injection quantity and therefore about various properties of the internal combustion engine and possibly also of a vehicle which is driven by said internal combustion engine.
In principle, the position of the blocking element and therefore the degree of opening of the injection valve can be detected by means of an additional sensor. Therefore, it is known, for example, to arrange a specific needle stroke sensor on the nozzle needle of an injector and to directly measure the needle stroke of the nozzle needle using this additional sensor. However, this leads to higher production costs for the injection valve. The only way that is known from the prior art to achieve this without an additional sensor is to measure the position of the blocking element at the time at which it reaches the blocking position, that is to say only during the so-called closing or blocking time point of the injection valve, as described in documents DE 10 2009 032 521 A1 and DE 10 2004 023 545 A1 for example.
However, this information on its own is not sufficient to allow a conclusion to be drawn about the degree of opening of the valve during the injection operation, that is to say before the closing time point.